1. Field of the Invention
The invention involves a process for removing defective semiconductor chips, mounted in a flip chip configuration, from an organic substrate, such as an organic printed circuit board, and replacing the defective semiconductor chips with functional semiconductor chips.
2. Description of the Related Art
One method for packaging a semiconductor integrated circuit device (hereinafter referred to as a semiconductor chip or just chip) involves attaching solder balls to contact pads on the circuit-bearing surface of the chip. These solder balls have compositions which include, for example, 97% (by weight) Pb and 3% (by weight) Sn, with the corresponding melting temperature being 320 degrees C. This chip is then placed face-down, in the so-called flip chip configuration, onto a ceramic, e.g., alumina, substrate, with the solder balls being placed onto solderable metal contact pads on the ceramic substrate. Sufficient heat is then applied to melt the solder balls. Upon cooling and re-solidification, the solder balls serve to mechanically and electrically connect the chip to the ceramic substrate, resulting in what is commonly referred to as a ceramic chip carrier. The ceramic chip carrier is then mounted onto a printed circuit board (PCB) or printed circuit card (PCC) using, for example, a lead frame.
Another method for packaging a semiconductor chip involves mounting the chip, in the flip chip configuration, directly onto a PCB or PCC, which is conventionally comprised of organic materials such as the epoxy resin/fiberglass compositions sold under the trade names FR4 and DriClad, as well as polytetrafluoroethylene (PTFE)-based materials. As before, solder balls are attached to contact pads on the circuit-bearing surface of the chip, the solder balls having compositions which include, for example, 97% (by weight) Pb and 3% (by weight) Sn. Again as before, the solder balls attached to the chip are to be attached to corresponding solderable metal contact pads on the PCB or PCC. However, in this instance, such attachment is not achieved by melting the solder balls because the organic materials comprising the PCB or PCC are incapable of withstanding the temperatures needed to achieve solder ball melting. Rather, a region of Pb-Sn solder having the eutectic composition, i.e., 63% (by weight) Pb and 37% (by weight) Sn, is first applied to each solderable metal contact pad on the PCB or PCC. The solder balls attached to the chip are then brought into contact with the eutectic solder on the PCB or PCC, and tile eutectic solder is then heated to its melting temperature of 183 degrees C., which has no adverse effects on the organic materials comprising the PCB or PCC. Upon cooling and re-solidification, the eutectic solder serves to mechanically and electrically connect the solder balls on the chip to the contact pads on the PCB or PCC.
When mounting a chip onto a substrate, the mounting process should preferably take into account the possibility of rework, i.e., the possibility that the chip may be defective, and will therefore have to be removed and replaced with a functional chip. In this regard, until recently, the solder balls on chips which have been mounted directly onto PCBs or PCCs have been arranged in grids having relatively large pitches, e.g., pitches of 14 mils. In addition, the solderable metal contact pads on the corresponding PCBs or PCCs have had relatively large dimensions, e.g., lengths and widths of, for example, 5 mils by 6 mils. This has permitted a relatively large amount of eutectic solder, e.g., 80 cubic mils of solder, having a mean thickness of 1 mil, to be applied to each solderable metal contact pad of a PCB or PCC when initially mounting a chip onto the PCB or PCC. Such an amount, while significantly more than is needed to achieve chip mounting, is advantageous because it permits ready rework. That is, if the mounted chip is found to be defective, in the conventional rework process, the eutectic solder connecting the chip solder balls to the contact pads of the PCB or PCC is heated to its melting temperature. Then, the defective chip, with its unmelted solder balls, is pulled off the PCB or PCC. This does result in the removal of some of the eutectic solder. However, the volume and mean thickness of the remaining eutectic solder is more than adequate to permit a new, functional chip to be directly mounted onto the PCB or PCC, without skewing or tilting the chip, using just the eutectic solder remaining on the contact pads.
Just recently, the pitches of the solder ball grids on chips to be directly mounted onto PCBs or PCCs have been reduced in size to, for example, 9 mils. In addition, the dimensions of the solderable metal contact pads on the corresponding PCBs or PCCs have also been reduced to, for example, 3 mils by 5 mils. For reasons discovered by the present inventors, discussed below, the conventional rework process, described above, is no longer useful for such relatively small solder ball grid pitches and/or relatively small contact pads. Instead, a new rework process has been developed by the present inventors, which is described below.